Neuroblastoma is the most common extracranial malignant solid tumor in children and drug resistance is a major reason for poor outcome of neuroblastoma. Thus, there is an urgent need for novel therapies. This proposal will pursue that goal by defining the role of. In our preliminary studies, we have found that DUSP26 is specifically overexpressed in majority of NB cell lines and tissue specimens. DUSP26 is required for neuroblastoma colony formation in vitro and promotes the resistance of neuroblastoma to doxorubicin- induced apoptosis by acting as a p53 phosphatase to inhibit p53 tumor suppressor function in neuroblastoma cells. Thus, DUSP26 acts as a p53 specific phosphatase to inhibit p53 functions in response to genotoxic stress in p53 wild-type neuroblastoma. Majority of neuroblastoma maintains functional p53 and apoptotic mechanisms which are suppressed in the majority of primary and relapsed cases. Thus, re-activation of p53 activity by inhibiting DUSP26 phosphatase activity represents an attractive therapeutic approach to this cancer. The central hypothesis of this work is that DUSP26 is an essential factor that regulates neuroblastoma growth, metastasis, and chemo-resistance. The proposed experiments will test this hypothesis by analyzing the biological role of DUSP26 in neuroblastoma development and determine whether DUSP26 inhibitor is able to inhibit neuroblastoma growth and enhance neuroblastoma chemo- sensitivity in an orthotopic mouse model. The specific aims of this application are: 1) to determine whether DUSP26 knockdown alters neuroblastoma tumor growth in an orthotopic mouse model; 2) to whether DUSP26 knockdown alters neuroblastoma tumor chemo-resistance in an orthotopic mouse model; 3) to determine whether a novel DUSP26 inhibitor alters neuroblastoma tumor growth in an orthotopic mouse model; and 4) to determine the mechanism of DUSP26 function in neuroblastoma in response to doxorubicin treatment. The proposed project, if successful, will establish DUSP26 as a novel therapeutic target in neuroblastoma and a small molecule inhibitor against DUSP26 phosphatase activity may offer a therapeutic benefit for treating neuroblastoma patients. The long-term goal of this proposal is to improve the outcome of neuroblastoma patients by identifying and validating potential novel druggable targets for therapeutic intervention of this devastating disease in children.